Cumulative fraction of afternoon hours of observed CO₂ mole fraction enhancement above a given level. a) CO₂ enhancement for all sites. Here enhancement is the difference between each site and the INFLUX network minimum for that hour. The averaging period is 1 January – 30 April 2013. Site details are listed in Table 1. Sites 01 and 09 are considered potential background sites. b) As in a), but for Site 01 and 09 when the wind is from the southeast (90–180°), from the southwest (180–270°), and from the northwest (270–360°). During this period, the wind comes from the northeast less than 8% of the time; thus the CO₂ enhancement from that direction is not considered. The results for Site 04 averaged over all wind directions are shown for comparison. DOI: https://doi.org/10.1525/elementa.127.f4